1. Field of the Invention
The present invention is directed to wireless communication systems and, more particularly, to the transmission of control information in the uplink of a communication system.
2. Description of the Art
A communication system consists of a DownLink (DL), supporting transmissions of signals from a base station (Node B) to User Equipments (UEs), and of an UpLink (UL), supporting transmissions of signals from UEs to the Node B. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other terminology.
UL signals consist of data signals, carrying the information content, control signals, and Reference Signals (RS), which are also known as pilot signals. The UEs convey UL data signals through a Physical Uplink Shared CHannel (PUSCH). The UL control signals include acknowledgement signals associated with the application of Hybrid Automatic Repeat reQuest (HARM), Service Request (SR) signals, Channel Quality Indicator (CQI) signals, Precoding Matrix Indicator (PMI) signals, or Rank Indicator (RI) signals. Any combination of HARQ-ACKnowledgement (HARQ-ACK), SR, CQI, PMI, or RI will be referred to as Uplink Control Information (UCI). UCI can be transmitted in a Physical Uplink Control CHannel (PUCCH) or, together with data, in the PUSCH over a Transmission Time Interval (TTI).
A UE transmits an HARQ-ACK signal in response to data packet reception in the DL. Depending on whether the data packet reception is correct or incorrect, the HARQ-ACK signal has an ACK or a NAK value, respectively. The UE transmits an SR signal to request UL resources for signal transmission. The UE transmits a CQI signal to inform the Node B of the DL channel conditions it experiences, enabling the Node B to perform channel-dependent scheduling of DL data packets. The UE transmits PMI/RI signals to inform the Node B how to combine the transmission of a signal to the UE from multiple Node B antennas in accordance with a Multiple-Input Multiple-Output (MIMO) principle. Any of the possible combinations of HARQ-ACK, SR, CQI, PMI, and RI signals may be transmitted by a UE jointly with data information in the PUSCH, or separate from data information in the PUCCH.
A structure for the PUSCH transmission in the UL TTI, which for simplicity is assumed to consist of one sub-frame, is illustrated in FIG. 1. A sub-frame 110 includes two slots. Each slot 120 includes NsymbUL symbols used for the transmission of data information, UCI, or RSs. Each symbol 130 further includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The PUSCH transmission in one slot may be in the same or a different part of the operating BandWidth (BW) as/than the PUSCH transmission in the other slot. Some symbols in each slot can be used for RS transmission 140 to provide channel estimation and to enable coherent demodulation of the received signal. The transmission BW is assumed to consist of frequency resource units, which will be referred to as Physical Resource Blocks (PRBs). Each PRB is further assumed to consist of NscRB sub-carriers, or Resource Elements (REs). A UE is allocated MPUSCH PRBs 150 for PUSCH transmission for a total of MscPUSCH=MPUSCH·NscRB REs for the PUSCH transmission BW. The last symbol of a sub-frame may be used for the transmission of a Sounding Reference Signal (SRS) 160, from one or more UEs, whose primary purpose is to provide a CQI for the UL channel that each of these UEs experiences.
A UE transmitter block diagram for UCI and data transmission in the same PUSCH sub-frame is illustrated in FIG. 2. Coded CQI bits and/or PMI bits 205 and coded data bits 210 are multiplexed in step 220. If HARQ-ACK bits also need to be multiplexed, data bits are punctured to accommodate HARQ-ACK bits in step 230. A Discrete Fourier Transform (DFT) of the combined data bits and UCI bits is then obtained in step 240. The REs corresponding to the assigned transmission BW are selected via sub-carrier mapping in step 250 through the control of localized FDMA in step 255. The Inverse Fast Fourier Transform (IFFT) is performed in step 260. The CP is inserted in step 270 and filtering is applied via time windowing in step 280 to achieve a transmitted signal 290. Additional transmitter circuitry such as a digital-to-analog converter, analog filters, amplifiers, and transmitter antennas are not illustrated. Also, the encoding process for the data bits and the CQI and/or PMI bits, as well as the modulation process for all transmitted bits, are omitted for brevity. The PUSCH signal transmission is assumed to be over clusters of contiguous REs in accordance to the DFT Spread Orthogonal Frequency Multiple Access (DFT-S-OFDMA) method allowing signal transmission over one cluster 295A (also known as Single-Carrier Frequency Division Multiple Access (SC-FDMA)), or over multiple non-contiguous clusters of contiguous BW 295B.
The Node B receiver performs the reverse (complementary) operations of those of the UE transmitter. This is conceptually illustrated in FIG. 3 where the reverse operations of those illustrated in FIG. 2 are performed. After an antenna receives the Radio-Frequency (RF) analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) which are not shown for brevity, a digital signal 310 is filtered via time windowing in step 320 and the CP is removed in step 330. Subsequently, the Node B receiver applies a Fast Fourier Transform (FFT) in step 340. The REs used by the UE transmitter are selected via sub-carrier demapping in step 350 under the control of reception bandwidth in step 345. An Inverse DFT (IDFT) is applied in step 360. The HARQ-ACK bits are extracted and respective erasures for the data bits are placed in step 370. Data bits 390 and CQI/PMI bits 395 are de-multiplexed in step 380. Well known Node B receiver functionalities such as channel estimation, demodulation, and decoding are not shown for brevity.
A structure for the PUCCH transmission in one slot of a sub-frame is illustrated in FIG. 4 for HARQ-ACK, SR, or RI transmission, and in FIG. 5 for CQI or PMI transmission. The transmission in the other slot, which may be at a different part of the operating BW for frequency diversity, has the same structure with the possible exception of the last symbol, which may be punctured to accommodate SRS transmission for the PUSCH. The PUCCH transmission for each UCI signal is assumed to be in one PRB.
Referring to FIG. 4, a HARQ-ACK (or SR, or RI) transmission structure 410 includes the transmission of HARQ-ACK signals and RS for enabling coherent demodulation of the HARQ-ACK signals. HARQ-ACK bits 420 modulate 430 a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence 440, for example with BPSK or QPSK modulation, which is then transmitted after performing the IFFT operation, as is subsequently described. Each RS 450 is transmitted through the non-modulated CAZAC sequence.
Referring to FIG. 5, CQI (or PMI) transmission structure 510 includes the transmission of CQI signals and RS. CQI bits 520 again modulate 530 a CAZAC sequence 540, for example using QPSK modulation, which is then transmitted after performing the IFFT operation. Each RS 550 is transmitted through the non-modulated CAZAC sequence.
An example of CAZAC sequences is given by Equation (1) below.
                                          c            k                    ⁡                      (            n            )                          =                  exp          ⁡                      [                                                            j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                  k                                L                            ⁢                              (                                  n                  +                                      n                    ⁢                                                                  n                        +                        1                                            2                                                                      )                                      ]                                              (        1        )            where L is the length of the CAZAC sequence, n is the index of an element of the sequence n={0, 1, . . . , L−1}, and k is the index of the sequence. If L is a prime integer, there are L−1 distinct sequences which are defined as k ranges in {0, 1, . . . , L−1}. If the PRBs consist of an even number of REs, such as for example NscRB=12, CAZAC sequences with even length can be directly generated through computer search for sequences satisfying the CAZAC properties.
FIG. 6 is a diagram illustrating a UE transmitter structure for a CAZAC sequence that can be used non-modulated as an RS or modulated as an HARQ-ACK signal or CQI signal using BPSK (1 HARQ-ACK bit) or QPSK (2 HARQ-ACK bits or CQI bits). The frequency-domain version of a computer generated CAZAC sequence is used in step 610. The REs corresponding to the assigned PUCCH BW are selected via sub-carrier mapping in step 630 under control of the transmission bandwidth in step 620. An IFFT is performed in step 640, and a CS is applied to the output in step 650 as is subsequently described. The CP is inserted in step 660 and filtering via time windowing is applied in step 670 resulting in transmitted signal 680. Zero padding is assumed to be inserted by the reference UE in REs used for the signal transmission by other UEs and in guard REs (not shown). Moreover, for brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas as they are known in the art, are not shown.
The reverse (complementary) transmitter functions are performed at the Node B receiver for the reception of the CAZAC sequence. This is conceptually illustrated in FIG. 7 where the reverse operations of those in FIG. 6 apply. An antenna receives RF analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) a digital received signal 710 is filtered via time windowing in step 720 and the CP is removed in step 730. Subsequently, the CS is restored in step 740, an FFT is applied in step 750. The transmitted REs are selected in step 760 via sub-carrier demapping under control of reception bandwidth in step 765. FIG. 7 also shows the subsequent correlation with the replica of the CAZAC sequence in step 780 by multiplier in step 770. Finally, output 790 is obtained which can then be passed to a channel estimation unit, such as a time-frequency interpolator, for an RS, or can to detect the transmitted information, for the CAZAC sequence modulated by HARQ-ACK information bits or CQI information bits.
Different CSs of the same CAZAC sequence provide orthogonal CAZAC sequences. Therefore, different CSs of the same CAZAC sequence can be allocated to different UEs in the same PRB for their RS, or UCI signal transmission and achieve orthogonal UE multiplexing. This principle is illustrated in FIG. 8. In order for multiple CAZAC sequences 810, 830, 850, 870, respectively generated from multiple CSs 820, 840, 860, 880 of the same CAZAC sequence to be orthogonal, CS value Δ890 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If Ts is the DFT-S-OFDM symbol duration, the number of such CSs is equal to the mathematical floor of the ratio Ts/D.
Orthogonal multiplexing for the HARQ-ACK PUCCH transmission structure can be achieved not only through different CS values of the CAZAC sequence, but also by applying orthogonal covering in the time domain. The HARQ-ACK and RS symbols in each slot are respectively multiplied with a first and a second orthogonal code. However, these multiplexing aspects are not material to the invention and further description is omitted for brevity.
When UCI is transmitted in the PUSCH, some REs that would otherwise be used for data are used for UCI, which usually requires better reception reliability than data. As a result more REs are required to transmit a UCI bit than a data bit. Additionally, UCI may require different reception reliability depending on their type. For example, the target Bit Error Rate (BER) for HARQ-ACK is typically much lower than that of CQI/PMI, since erroneous reception of HARQ-ACK has more detrimental consequences and, due their small number, HARQ-ACK bits are protected through repetition coding while more powerful coding methods can apply to CQI/PMI bits. The number of REs required for UCI transmission in the PUSCH is proportional to the spectral efficiency of the data transmission as determined by the data Modulation and Coding Scheme (MCS). For a certain target data BLock Error Rate (BLER), the MCS depends on the Signal to Interference and Noise Ratio (SINR) the UL signal transmission experiences. As the Node B scheduler may vary the target data BLER, it may configure an offset for the number of REs of each UCI signal in order to avoid having only an exclusive link with the data MCS.
Among the UCI signals, HARQ-ACK signals have the highest reliability requirements and the respective REs are located next to the RS in each slot in order to obtain the most accurate channel estimate for their demodulation. The number of coded symbols Q′ for the HARQ-ACK (or RI) transmission in the PUSCH can be determined as set forth in Equation (2).
                              Q          ′                =                  min          (                                    ⌈                                                O                  ·                                      β                    offset                                          ACK                      /                      NAK                                                                                                            Q                    m                                    ·                  R                                            ⌉                        ,                          4              ·                              M                sc                                  PUSCH                  -                  current                                                              )                                    (        2        )            where O is the number of HARQ-ACK (or RI) bits (for example, 1 or 2), βoffsetACK/NAK is the offset configured to the UE by the Node B, and MscPUSCH-current is the PUSCH BW in the current sub-frame. Qm and R are respectively the number of bits for data modulation (Qm=2, 4, 6 for QPSK, QAM16, QAM64, respectively) and the data code rate of the initial PUSCH transmission for the same transport block. The link between HARQ-ACK REs and data MCS is through Qm·R. The code rate R is defined as
  R  =            (                        ∑                      r            =            0                                C            -            1                          ⁢                  K          r                    )        /          (                        Q          m                ·                  M          sc          PUSCH                ·                  N          symb          PUSCH                    )      where NsymbPUSCH=(2·(NsymbUL1)−NSRS) with NSRS=1 if SRS transmission at least partially overlaps with the PUSCH BW and NSRS=0 otherwise. Finally, C is the total number of code blocks and Kr is the number of bits for code block number r. The number of HARQ-ACK (or RI) REs is limited to the ones corresponding to 4 DFT-S-OFDM symbols per sub-frame (2 symbols per slot). A similar expression applies for the number of coded CQI/PMI symbols per sub-frame which is omitted for brevity. The principle of the linkage of the UCI resources to the data MCS and the assigned UCI offset is described above for the HARQ-ACK (or RI) symbols.
There are several reasons for UCI to be in the PUSCH, when it occurs with data in the same sub-frame, and not in the PUCCH. A first reason is that concurrent transmission of data in the PUSCH and UCI in the PUCCH increases the Peak-to-Average Power Ratio (PAPR) or the Cubic Metric (CM) of the combined signal transmission, which then requires higher transmission power in both PUCCH and PUSCH to achieve the same reception reliability as when only one of the PUSCH or PUCCH is transmitted. This increase in power increases interference and may not even be possible for power limited UEs. A second reason is the UCI payload may not be possible to transmit in the PUCCH. For example, for the CQI transmission structure in FIG. 5, only 20 coded CQI symbols can be transmitted per sub-frame and therefore, detailed CQI reports need to be sent through the PUSCH.
While UCI and data transmission in the PUSCH preserves the single-carrier property and avoids increasing the CM of PUSCH transmission, it is not spectrally efficient as PUCCH resources assigned to UCI transmission remain unused. Also, multiplexing UCI in the PUSCH may often result in an excessive number of REs being used for UCI instead of data.
In addition to concurrent transmission of UCI and data, concurrent transmission of various UCI signals may often occur. For example, HARQ-ACK and CQI transmission may need to occur in the same sub-frame in the absence of data transmission from a UE. To preserve the single-carrier property and avoid increasing the CM of concurrent transmissions of HARQ-ACK and CQI signals, multiplexing of the two can be in the same PUCCH. For example, HARQ-ACK transmission can be multiplexed in the PUCCH structure of FIG. 5, which is used for CQI transmission by scaling the second RS in each slot by “−1” if ACK is transmitted, and by “+1” if NAK is transmitted, as is the case in 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). However, the PUCCH assigned to HARQ-ACK transmission remains unused and the HARQ-ACK BER may degrade relative to when the HARQ-ACK signal transmission uses its own PUCCH resources.